"Oh man, that's scary," Sugar Ray Leonard says as his eyes open wide and he clutches my arm. "That hits me like a thunderbolt. It's amazing how the truth gets you like that."

Even when he is spooked, Leonard still looks like a million dollars. The handsome face and the gleaming smile remain, even if they are worn by the years of pain. Beyond his epic fights in the 1980s against Thomas Hearns, Roberto Durán and Marvin Hagler, Leonard has waged lonelier battles. The 55-year-old struggled for decades with the secret fact that he had been sexually molested twice as a boy by middle-aged men; while his more recent trials with drugs and alcoholism crossed bleak terrain.

Yet this is how an involving interview with one of the greatest fighters in history unfolds, as Leonard reacts with raw immediacy. "Tell me again what he said," he urges.

And so I recount in more detail how, years ago in Las Vegas, Hugh McIlvanney, the great boxing writer, who then worked for the Observer, listened patiently while I waffled on about Mike Tyson's "bad intentions". McIlvanney smiled sagely and asked what I thought of the retired Ray Leonard. I'd only seen Leonard fight on television and I wittered some more about Sugar's sublime artistry. McIlvanney cut to the core. He spoke vividly of the hard chip of ice that Leonard stored in his fighting heart. Sugar Ray must have endured terrible darkness to fight with such chilling brilliance.

"It's so true," Leonard says. "That's exactly how it felt. Every time I stepped into the ring I had that chip of ice. My brother, Roger, saw it when we were sparring. I moved towards him and he said, 'Stop, Ray! Look at your fucking eyes. You look like you wanna kill me!' Maybe I used what happened to me outside when I ducked inside the ropes.

"But it was subconscious. Did I fight so hard because I was distressed by the sexual abuse? Not to my knowledge. Outside, I'm not a confrontational guy. Even if I'm used to talking on TV I'm actually reserved and quiet – almost shy. But I could be a mean guy in that ring because I felt confident."

Leonard looks up as sunshine streams in through his London hotel window. "I went through real darkness but the ring was my light. That was the one place I felt safe. I could control what happened in the ring. My heart turned icy."

In his gripping and revealing autobiography, Leonard strips bare the gilded image that once defined and separated him from the street menace that Durán, Hearns and Hagler all brought to their violent trade. An Olympic gold medal winner, to whom the trainer Angelo Dundee hitched his stool as the only possible successor to Muhammad Ali, Leonard kept his real life secret. The raw truth is out now.

Even after his victory in the 1976 Olympic Games, Leonard nearly abandoned boxing. He tells an upsetting anecdote of how close he came to giving into heroin just months after he won gold in Montreal. "I wanted to be like Bruce Jenner," he says of the flaxen-haired American who won the decathlon at those same Olympics. "But he was white and just weeks after the Games I felt like a nigger again."

A despairing Leonard ended up in an apartment where a bereft group of black men injected each other with heroin. He begged them: "Hit me, man, hit me."

Leonard shows me his arm and remembers how one of the junkies wrapped a cord around him to make his vein rise up. The old fighter narrows the gap between his thumb and index finger to a few millimetres. "I was that near the needle. But one guy stepped in. He said: 'Hey, Ray, don't fuck your life up. You're the champ, man.' These guys were drifting off into never-never land but they still had enough inside to say, 'No, Ray!'"

He succumbed years later to cocaine and drink but, before then, as Leonard says wearily, "I went on this tumultuous rollercoaster ride. I came from nothing and achieved humungous fame and fortune. But I worked hard. I had discipline and determination. I had that ice in me."

Ali was one of the first to recognise Leonard's extraordinary resolve. "I couldn't believe it," Leonard says. "Not long after that scene with the needle I got invited to Ali's third fight against Ken Norton [in September 1976]. Norton had won the first, Ali the second. A tough night lay ahead for Ali.

"But he asked to see me in his dressing room before the fight. It was mind-boggling that Ali could think about me turning pro just minutes before the bell. He said: 'Make sure no one owns you. Be your own man.'"

Leonard sidestepped the controlling web of Don King and Bob Arum and forged a deliberately independent path. He relied on a white lawyer, Mike Trainer, to guide him through the promotional minefield and followed Ali's instruction as he made and kept most of his millions. Yet Leonard's legacy burns most fiercely in the ring.

We go over the old fights again, Leonard sweeping aside my reticence to travel down the beaten track once more. He shows me how much he still relishes talking about Durán, Hearns and Hagler by reliving those nights in graphic and intricate detail. Leonard pulls off the trick of making me believe his stories are still as riveting to him as they are to me. Days later, some still linger in my head.

"Joe Frazier said Roberto Durán looked like Charles Manson," Leonard cackles. "And he did. God, Durán was so violent, so mean. Some people out there, in the streets, can shoot you in the face and then go get some lunch. They have no conscience. Durán was like that. It freaked me out because Durán was so nasty. He got to me the first time."

Leonard lost his unbeaten record to Durán in June 1980 – in a clearcut decision. But, five months later, Leonard came to the rematch with a different plan. "My brother Roger gave it to me. This was drugged-up Roger. In the gym he said: "You have to piss him off.' I said: 'You gotta be kidding?' But Roger kept telling me. 'You gotta embarrass this guy. You gotta make him mad with you.'

"In the rematch with Durán I was boxing smart and, then, I started fooling around. I threw those crazy bolo punches and the audience was laughing. Really laughing. I could see in Durán's eyes he didn't like it. He was a bad bully and here I was ridiculing him. Eventually, he just quit. He threw up his hands in frustration and walked away. He never realised the ramifications it would have on the rest of his life. But that fight should not dictate his legacy. Durán was a great fighter."

Leonard speaks here with real intent; and he now feels almost tender towards Durán. "I remember seeing him just as we left the arena that night. I was in my car. He was being driven in the back of his car. What are the chances of that happening – after one of the biggest fights in history? I thought, 'Shit, Durán.' I waved to him. He lifted his hand to me – but there was no life in him.

"It still eats away at him today. I don't think a day goes by when he's not reminded of it. I know I'm asked about it every day because it's one of those moments in sporting history that fascinates people.

"What happened to Roberto Durán? He hears that question every day and I feel for him. I respect and like him now. But I know he won't address it. He won't go there. It's just like I wanted to avoid the sexual abuse so long."

Leonard defeated Durán again, and he beat Hearns and luckily drew their second fight. He also shocked the world by coming out of retirement in 1987 to icily steal a decision from the ferocious Hagler – who has yet to forgive him. But, more than those momentous contests, Leonard's battles outside the ring now define him. Like so many fighters, he felt lost when his boxing prowess slipped away. He still had money to burn, and a huge posse of fawning acolytes and groupies around him, and Leonard began to use cocaine as a way of pursuing the intensity he had felt as a boxer.

"I also started to hang around the sportswriters," he says, wryly, "because I was doing television for HBO. Those guys would always say, late at night, 'C'mon, Sugar Ray, have one more…' The drugs came first but the alcohol became the major problem.

"My [second] wife, Bernadette, was concerned about all these revelations – the abuse, the drugs, the drinking. When I started the book we had one child in the fourth grade and another in the sixth grade. My wife worried because she was thinking from a wife's and a mother's standpoint and trying to protect us. But if I had kept suppressing it, it would've finally killed me. I don't mind that, talking about it, I sometimes cry. I feel such lightness now it's out."

When Leonard attended his first AA meeting six years ago he was instantly recognised. "You could tell I was a newcomer because I said, 'Hi, my name's Ray Leonard.' They said, 'Just your first name, Ray, that's all we need. It took a long time before I could say, 'Hi, I'm Ray and I'm an alcoholic.' I got there in the end. But, even here, I open up the mini-bar and I think about it a little. One part of my head says, 'Come on, you can do that.' The other shuts it down. I stay strong.

"You know, my dad turns 90 in June. He tells me the same stories every time I go to the house. Five minutes later, he starts telling them all over again. But he's old. He's been married 62 years to my mother.

"I used to find it hard to go there and so I never went unless I had drink or some substance in my body. I cushioned myself. But that's no way to live. Now, I'm straight every time I go home.

"It's hard – because we all know how brutal life gets. But I felt something special when I was recently there. We were sitting and having coffee and I said, 'Daddy, you know I'm here and I'm still not drinking. I haven't drunk for years now.' He looked up at me and said, 'Son, I'm so proud of you.' That felt so good.

"It's been a hell of a ride but I got off that rollercoaster in one piece. I wouldn't change anything because the mistakes and the hurt are as important as all the great fights. They made me who I am today. Sugar's still there, in the background, but Ray's here now. I'm just Ray Leonard. It's as simple, and as sweet, as that."